In this article, a novel digital dual-loop interleaving control algorithm is proposed for an asymmetric multiphase buck converter to further enhance the transient response under the large load step and ultrafast slew rate. The proposed dual loop consists of a novel nonlinear average current loop and a digital integration constant on -time (DICOT) controlled voltage loop, which are interleaving connected. The normal phases with average current control aim to deliver the majority of power, while the auxiliary phases with DICOT control are designed to regulate the output voltage and for the fast transient response. With the interleaving control approach, the average inductor current of auxiliary phases remains unchanged in a steady state under different load conditions and that of normal phases follows the load current level. Compared to existing researches, the proposed digital dual-loop interleaving control has robust operation in auxiliary phases and flexibility control of normal and auxiliary phases, as well as the simple hardware implementation. The proposed algorithm is constructed in field-programmable gate array and is applied on the eight phases asymmetric buck system, the maximum load step of 250 A with the current slew rate of 1200 A/μs is tested, the undershoot and overshoot are 30 and 41 mV respectively, and the recovery times are around 20 μs.